The present disclosure relates generally to organic films, and more particularly to films for detecting fluoride.
Chemical sensors, including electrochemical (potentiometric ion-selective electrodes, conductometric sensors, and the like) and optical sensors, may be designed with selective response to given cations using various natural antibiotics (e.g., valinomycin), crown ethers, calixarenes and other ionophore-type structures within thin organic liquid or polymeric membranes/films. However, the design of analogous anion selective sensors has been far more difficult, due, at least in part, to the lack of suitable lipophilic host molecules that exhibit specificity for interaction with given anions. Further, organic membrane/film type sensors with selective response to fluoride, in particular, may be difficult to achieve because of the negative Gibbs free energy of hydration of this anion relative to others (−436 kJ/mol). Hence, achieving fluoride extraction into low dielectric constant organic liquid or polymeric films with selectivity over anions with much more positive free energies of hydration (e.g., perchlorate, −201 kJ/mol) may require an exceptionally high degree of anion binding discrimination by the host molecule to overcome the favorable single ion partition coefficients associated with such interferent anions.
A variety of metal-ligand complexes, for example, metalloporphyrins and related structures, have been examined as potential ionophores in organic membranes/films to prepare useful anion sensors using both optical and potentiometric modes of detection. Selectivity is achieved based on the relative binding interactions of anions as axial ligands with the metal ion-ligand complexes within the organic films. Films doped with gallium(III) and zirconium(IV) porphyrins display enhanced selectivity for fluoride ions, although, in some instances, the sensitivity is not adequate for certain applications (e.g., detecting fluoride levels in municipal drinking waters). Further, the selectivity over other anions (e.g. thiocyanate, perchlorate, salicylate, and iodide) may be marginal for practical applications.
As such, it would be desirable to provide a film and sensor having relatively high selectivity for fluoride ions.